A common method of increasing productivity of a hydrocarbon-bearing formation penetrated by a well is to subject the formation to stimulation techniques, such as hydraulic fracturing. Fracturing of a subterranean formation is accomplished by pumping a fracturing fluid into the well which penetrates the formation at a sufficient pressure and flow rate such that cracks or fissures are created or enlarged in the surrounding formation. The fracturing fluid typically contains a proppant which functions to prop open fractures such that the fracture provides larger flow channels through which an increased quantity of a hydrocarbon may flow. Productive capability of the well is therefore increased.
In a typical hydraulic fracturing treatment operation, a viscosified fracturing fluid is used which contains at least one water-soluble polymer which has been hydrated in water and which has been chemically modified with a crosslinking agent in order to increase fluid viscosity.
Certain subterranean formations subjected to hydraulic fracturing are however water sensitive. For instance, formations rich in swellable and migrating clays are water sensitive due to the presence of kaolinite, chlorite, illite and mixed layers of illite and smectite.
Further, fracturing fluids which contain water can damage some well formations due to adverse water saturation effects, which can include sub-irreducible water saturation. Water retention issues may be especially acute in tight gas formations which are water-wet and under-saturated where the initial water saturation in the reservoir is less than the capillary equilibrium irreducible water saturation. When exposed to aqueous based fluids, these formations will trap water for long periods of time, if not permanently, especially in the near-wellbore region of the well. The saturation of the formation with water can result in reduced permeability to hydrocarbons, which in turn can cause reduced productivity of the well.
In addition to the increased potential for formation damage from imbibed water, interest in non-aqueous fracturing methods has increased in light of the unavailability of water at the wellsite. Further, certain formations, such as shale, flow back 25 to 40% of fracturing water which has to be handled prior to putting the produced gas on pipeline. Non-aqueous fracturing fluids minimize the concern of flowback water.
In the past, liquid carbon dioxide has been used as a non-aqueous fracturing fluid as well as liquid petroleum gas (LPG). Carbon dioxide, however, is not available in all locations. In addition, after completion of the frac job, the amount of carbon dioxide in the produced fluid typically has to be minimized before produced fluids can start to flow through the pipeline. LPG based fracturing fluids have been operationally successful; however, they carry inherent safety issues.
Alternative non-aqueous fracturing fluids have therefore been desired which minimize environmental and safety risks and which address the complexities presented during fracturing procedures.